Pneumatic balancers are in widespread use in the material handling industry. The balancers are formed of a large air cylinder that rotates about a fixed axis, reeling a wire rope in or out as the cylinder is pressurized with air. There are several advantages to this type of pneumatic lifting device. One advantage is that once a heavy load has been lifted, the load xe2x80x9cfloatsxe2x80x9d or is xe2x80x9cbalancedxe2x80x9d by the air that is pressurized inside the cylinder. This lets the user maneuver the heavy load up or down a short distance, while the load is positioned and secured onto an assembly. The combination of heavy lifting capacity and load suspension or xe2x80x9cbalancingxe2x80x9d makes pneumatic balancers useful.
One of the drawbacks with pneumatic balancers involves controlling the speed of pressurization of the cylinder. If the cylinder is pressurized at a relatively slow rate, the load is lifted at a slow rate which slows down the operator. If the cylinder is pressurized at a relatively high rate, a smaller or lighter load can be violently xe2x80x9claunchedxe2x80x9d across the room.
A traditional, all mechanical, solution to this problem is to use thumb screw adjustable needle valves to set the rate at which the cylinder pressurizes, and to employ a mechanical (centrifugal) brake which grabs and stops the cylinder if the velocity of the wire rope is too high. This solution, however, has drawbacks. The needle valves must be set based on the weight of the load being lifted. If the needle valves are set to lift a very heavy load by a first user, and at a later point a second user operates the balancer to lift a very light load without adjusting the needle valves, operation of the balancer causes the wire rope to accelerate much too quickly. At some later point, the mechanical brake xe2x80x9cgrabsxe2x80x9d the cylinder, stopping it instantly. This violent series of events can still xe2x80x9claunchxe2x80x9d the light load, and in any case, is unsettling to the unexpecting user.
Another solution to this problem involves the use of an electronic control system to control the rate at which the load is lifted. The control system requires a device to measure the speed of the wire rope, a valve which controls the speed of air entering the cylinder, and a device which allows the user to set the rate at which the load is being lifted. A drawback to the use of the electronic control system involves requiring the control system to exhibit a desirable response to a user""s speed request inputs over a full range of loads.
Classical control system theory assumes the use of a setpoint variable, which is, for example, a user""s joystick setting that represents the desired velocity of the wire rope, and a process variable, which is the actual velocity of the wire rope. The difference between the set point variable and the process variable is the process error. The process error is then differentiated and integrated with respect to time. These three representations of the error are then scaled with three empirically determined constants, and the resulting three products are summed, generating an output which is used as the driving function for the process. These three parts of the driving function are referred to as Proportional, Integral and Differential (PID). This entire function, which occurs in real time, feeds back process variable information into the function such that the process error is driven to zero. This type of control is referred to as a closed loop control.
One issue in using a PID control with a balancer is that the xe2x80x9cempirically determined constantsxe2x80x9d (or PID settings) are nominally set using a nominal weight on the balancer. This concept does not insure desirable performance over the full range of expected loads. Another complication is that the pressurized air, which is lifting the load, acts like a large spring (i.e. the air is compressible) where the spring constant changes depending on the weight being lifted (pressure in the cylinder).
A preferred embodiment of the present invention relates to a hoist whereby the lifting or lowering velocity of the hoist is independent of the load attached to the hoist.
More specifically, a velocity and acceleration limiter has been implemented within a hoist to decouple velocity and the effect of the load.
The user""s force sensitive inputs can control the pneumatic valve directly, i.e. with a fixed gain) as long as the wire rope""s velocity and acceleration are below pre-determined maximum values. If either the velocity or acceleration approaches the maximum value, the gain between the user""s inputs and the pneumatic valve is automatically reduced using a gain control or gain reduction algorithm, thus opening the pneumatic valve less and reducing the velocity or acceleration of the hoist.
In order to cause the control system to exhibit a desired response to a user""s speed request, the velocity and/or acceleration of the wire rope of a hoist is limited, rather than controlled, by the gain control algorithm. The user controls the velocity of the hoist in an open loop fashion. One benefit of the velocity limiting function is that as long as the user maintains the velocity below the maximum allowed, the output of the speed setting joystick is essentially connected directly to the input of the spool valve, with minimal effect of the gain control algorithm between the joystick and the hoist. This provides a direct feel for the user when moving a load at a slow rate of speed. A second benefit is that a velocity limiting function is not a closed loop function. There is no xe2x80x9csetpoint variablexe2x80x9d and there is no xe2x80x9cprocess errorxe2x80x9d that is driven to zero, which are fundamentally part of a closed loop system.
In one embodiment, the hoist includes a housing having a first end wall and a second end wall, the housing, first end wall and second end wall forming a chamber. An inlet mechanism is attached to the housing the inlet mechanism and allows the passage of a fluid, such as air, into the chamber. The hoist includes a piston mounted within the chamber and a valve connected to the inlet mechanism. The valve controls the amount of fluid entering the chamber. The hoist also includes a pressure sensor attached to the chamber and a position measuring device connected to the housing. The pressure sensor is in fluid communication with a fluid within the chamber and the position measuring device is in positional communication with the piston. Also included is an actuator in electrical communication with the valve, the actuator controlling the positioning of the valve, and a control system in electrical communication with the valve, the pressure sensor, the position measuring device and the actuator. The control system has a variable gain between the actuator and the valve, where the gain is reduced as the velocity or acceleration of a load attached to the piston approaches a preset maximum value.
The variable gain of the control system includes a gain reduction algorithm. This gain reduction algorithm preferably includes a square root function.
The actuator has a plurality actuator control inputs. Engagement of an actuator control input pulses the valve in a first direction. The actuator also includes a deadband value. Engagement of the deadband value after engagement of an actuator control input pulses the valve in a second direction, opposite to the first direction, which stops the motion of the piston. Preferably, the control system includes a pulse magnitude algorithm that scales a moving average of a valve magnitude upon engagement of the deadband value, based upon first direction of the valve.
The hoist can also include a load selector that adjusts a maximum allowed velocity value and a maximum allowed acceleration value of the hoist with respect to a load attached to the hoist. The control system of the hoist can also include a closed loop control system. The closed loop control system can include a position control or a pressure control.
An embodiment of the present invention also relates to a method for adjusting the velocity of a load attached to a pneumatic hoist. Another embodiment of the invention also relates to a method for stopping the motion of a load attached to a hoist